On the ionization ratio of the local interstellar medium

The abundance ratio of neutral hydrogen to neutral helium, as deduced from interplanetary observations of Lyman-alpha and He 584 A radiation by Mariner 10, is significantly lower than the cosmic abundance ratio of these elements, thus showing that the local interstellar medium (LISM) is partly ionized. It is shown that an important source of ionization of the LISM can be thermal collisions, yielding an ionization degree of about 50% for the hydrogen component.